The present invention is for a method for producing a catalyst for use in the high temperature shift water-gas shift reaction, and for the catalyst produced by the inventive method. The catalyst of the present invention comprises iron and at least one promoter. The catalyst is prepared via a method which comprises the preparation of a high purity iron precursor and which uses a nominal amount of water in the catalyst production. The catalyst particles prepared with the high purity iron precursor are essentially free of contaminants, and have essentially spherical particle shape and a relatively small particle size distribution range.
Hydrogen is an indispensable component for many petroleum and chemical processes. For example, refineries in the petroleum industry and methanol and ammonia plants in the chemical industry consume considerable quantities of hydrogen for the production of gasoline and fertilizers. Traditionally, the hydrogen for these reactions was produced as a by-product of other chemical reactions. However, as the environmental regulations demand cleaner, renewable and nonpolluting processes and products, most of the hydrogen balances at petroleum refineries are going negative. At the same time, H2 consumption is increasing because H2 is required to decrease the level of aromatics and sulfur in fuels.
Synthesis gas (syngas, a mixture of hydrogen gas and carbon monoxide) represents one of the most important feedstocks for the chemical industry. It is used to synthesize basic chemicals, such as methanol or oxyaldehydes, as well as for the production of ammonia and pure hydrogen. However, synthesis gas produced by steam reforming of hydrocarbons is typically not suitable for industrial applications because the syngas produced is relatively carbon monoxide rich and hydrogen poor.
In commercial operations, a water gas shift (WGS) reaction (Eq. 1) is used to convert carbon monoxide to carbon dioxide.CO+H2OCO2+H2 H=−9.84 Kcal/mol at 298° K  (Eq. 1)
An added benefit of the WGS reaction is that hydrogen is generated concurrently with the carbon monoxide conversion.
The water gas shift reaction is usually carried out in two stages: a high temperature stage, with typical reaction temperatures of about 350° C.–400° C., and a low temperature stage, with typical reaction temperatures of about 180° C.–240° C. While the lower temperature reactions favor more complete carbon monoxide conversion, the higher temperature reactions allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature reaction unit for bulk carbon monoxide conversion and heat recovery, and a low temperature reaction unit for final carbon monoxide conversion.
The commonly used catalysts for the water gas shift reaction at low temperature (referred to as a low temperature shift or LTS reaction) contain copper oxide, zinc oxide and aluminum oxide. Because these catalysts operate at relatively low temperatures, they generate equilibrium carbon monoxide concentrations of less than about 0.3% in the exit gas stream. However, the performance of the LTS catalyst to effect carbon monoxide conversion and the hydrogen yield gradually decrease during normal operations due to deactivation of the catalyst. This deactivation is caused by poisoning, generally from traces of chloride and sulfur compounds in the feed, or sintering from the hydrothermal environment of the reaction.
Chromium-promoted iron catalysts are normally used in the first stage high temperature reactions (referred to as a high temperature shift or HTS reaction) to effect carbon monoxide conversion at temperatures above about 350° C. and to reduce the CO content to about 3%–4% (see, for example, D. S. Newsom, Catal. Rev., 21, p. 275 (1980)). As is known from the literature, the chromium oxide promoter serves two functions: it enhances catalytic activity and it acts as a heat stabilizer—increasing the heat stability of magnetite, the active form of the catalyst, and preventing unduly rapid deactivation of the catalyst under conditions of technical use. More specifically, a typical composition of high temperature shift catalyst comprises from about 60 wt % to about 95 wt % Fe2O3, from about 0 wt % to about 20 wt % Cr2O3, from about 0 wt % to about 10 wt % of CuO and from about 0 wt % to about 10 wt % other active components such as ZrO2, TiO2, Co3O4, Al2O3, SiO2 and/or CeO2.
The HTS catalyst is usually made by either co-precipitation of metal salts (nitrate, sulfate, or acetate), thermal decomposition of metal complexes, or impregnation of metal salt onto a carrier. Depending on the preparation conditions (pH, temperature, addition rate and composition), one or several of the mixed iron precursors such as goethite, ferrihydrite, and/or lepidocrocit, may be present. The HTS catalyst is washed to remove foreign ions, and then is dried and calcined at a predetermined temperature to form oxides. With appropriate precursors and preparation conditions, a mixture of goethite and maghemite phases is formed during calcination at 250° C.–600° C. Thermodynamically, hematite is a more stable at higher temperatures. The goethite phase can be directly transformed to hematite at the temperatures higher than 300° C. Likewise, the maghemite phase is gradually converted to hematite at increased temperatures. The presence of both maghemite and hematite in a fresh catalyst precursor are critical to the activity of iron oxide commercial catalyst, and in a typical commercial catalyst calcined under mild temperature, the maghemite to hematite ratio is generally varies from 1 to 0.5.
A hydrothermally stable HTS catalyst is preferably formulated in such a way that the iron is stabilized by other components of the catalyst, such as chromium or aluminum oxide. For example, Cr substitution in an iron oxide lattice in a reduced working catalyst results in the expansion of the tetrahedral sites and the contraction of the octahedral sites, along with the oxidation of some Fe2+ to Fe3+. The resulting octahedral cations become more covalent in nature. In the presence of significant partial pressures of steam as found in the high temperature water gas shift reaction conditions, chromium oxide migration and/or inclusion of stabilizers into the iron oxide lattice inhibits crystallite growth.
Precipitated iron catalysts are generally regarded as superior high temperature water gas shift catalysts to the other types of iron catalysts described herein. The major disadvantages of the manufacture of precipitated iron catalysts include high cost, the method is labor intensive, and the by products are deleterious to the environment. Iron (ferric or ferrous) sulfate is the preferred iron source of precipitated iron catalysts because of availability and economics. But sulfur contamination from iron sulfate can have a deleterious effect on the activity of the resulting HTS catalyst. Further, the precipitation method tends to result in the formation of very viscous and gelatinous iron hydroxide or iron oxyhydrate precursor. This viscous precursor can be very difficult to filter and wash.
A process to produce iron-based HTS catalysts that reduces or eliminates the washing and filtration steps and has minimal emissions to the environment would be favorable. A logical process from a commercial viewpoint would be to promote, form, dry and calcine a commercially available iron oxide that has high purity and high surface area. Commercial iron oxides are readily available; however, they are usually prepared by treatment of steel with hydrochloric acid or sulfuric acid. These iron oxides contain significant amounts of impurities including chloride and sulfur which makes them unusable as raw materials for HTS catalysts. As is known in the art, the impurities of the commercial iron oxides (red or yellow iron oxides) can be reduced to very low level by the pickling process under very high temperatures. However, because of the extreme conditions of the pickling process, the surface area of the iron oxide is generally less than 10 m2/g making the iron oxide unsuitable for catalyst applications.